Gelatin or other hydrophilic polymers are commonly used as binders in image recording materials such as silver-based photographic materials and ink-jet receivers. These products are known to be swellable when in contact with water. The swelling property is essential in order to accomplish photographic processing chemistry or to absorb ink to generate images in the final product. However, the same properly also inhibits end users from fully enjoying the imaged product. It is worrisome that, during handling, the imaged product is liable to damage from spilled drinks, fingerprints, and the like.
Various kinds of processes for applying protective overcoats onto image recording processes are known. Teachings on various methods and apparatus for applying a controlled amount of overcoat material onto silver-based photographic materials during photographic processing have been disclosed, for example, in U.S. Pat. No. 5,984,539, U.S. Pat. No. 5,905,924, U.S. Pat. No. 5,875,370, and U.S. Pat. No. 6,087,051. It would be advantageous to implement overcoat technology, including material compositions, in currently existing photofinishing systems and laboratories with minimal or no changes. The temperature and residence time of photographic materials in the drying section of photofinishing trade equipment typically vary from 50.degree. C. to 70.degree. C. and from 30 seconds to 2.5 minutes. (The actual temperature of an image recording material during drying is lower than the temperature set for the dryer due to the evaporation of water.) High temperatures or fusing steps are not normally used in photoprocessing and would involve an additional expense and inconvenience if required to apply an overcoat composition to an imaged element.
U.S. Pat. No. 2,719,791 describes the use of an aqueous dispersion of an organic plastic material, which yields a water impermeable coating on drying. However, it is known that when dispersions of low Tg material (Tg&lt;25.degree. C.) are used to obtain a water resistant protective coating, the surface of the protective coating has an undesirable tacky characteristic. In customer's hands, such a coating on an image recording element can degrade due to print blocking, fingerprinting, dust attraction, and high scratch propensity. On the other hand, when dispersions of high Tg materials (Tg&gt;25.degree. C.) are used, it is difficult or impossible to form a continuous water resistance layer on the prints under the drying condition described above.
U.S. Pat. No. 5,376,434 describes the use of at least two resins in the protective overcoat layer of a photographic print, at least one first resin having a glass transition temperature (Tg) of not less than 80.degree. C., and at least one second resin having a Tg of 0.degree. C. to 30.degree. C., wherein an arithmetic mean of the glass transition temperatures of said first resin and said second resin is 30.degree. C. to 70.degree. C. The patent teaches the use of the high Tg resin to reduce the stickiness of the overcoat due to the low Tg material.
U.S. Pat. No. Patent 5,447,832 describes coating compositions for imaging elements comprising aqueous-based mixtures of lower Tg, film-forming polymeric particles and higher-Tg, non-film-forming polymeric particles. The film-forming particles provide continuous film formation and the non-film-forming particles comprise glassy polymers to provide resistance to tackiness, blocking, ferrotyping, abrasion and scratching.
While recognizing the above-mentioned benefits of two-component aqueous dispersions cited in U.S. Pat. Nos. 5,376,434 and 5,447,832, U.S. Pat. No. 5,952,130 further disclosed preferred substituents on the high and low Tg components in two-latex formulations in order to obtain improved fingerprint resistance. This patent discloses an overcoat comprising a first water-insoluble polymer having a Tg less than 25.degree. C. (film-forming) and a second water insoluble polymer having a Tg greater than 25.degree. C. (non-film forming), wherein one or the other of the two polymers comprise 20-100 weight percent of a monomer having a formula in which a substituent is Cl, F, or CN. Examples of such monomers are acrylonitrile, vinylidene chloride, tetrafluoroethylene, and methaciylonitlile.
U.S. Ser. No. 09/354,209 disclosed a low Tg polymer that fulfills the requirements of film formation, high gloss, excellent dry and wet scratch resistance, water resistance, oil and fingerprint resistance, and low tackiness without the use of a high Tg polymer latex. The polymer latex of this invention comprises 75% to 100%, preferably 80% to 95%, of the ethylenically unsaturated monomers of a certain formula wherein a substituent X is selected from the group consisting of --Cl, --F, or --CN, such as vinylidene chloride or acrylonitrile. A disclosed advantage of this coating is its ability to form a coating film at a modest drying temperature without being tacky on handling. The material composition is a colloidal dispersion of water insoluble polymeric materials having a Tg not more than 30.degree. C. Preferred monomers are acrylonitrile, methacrylonitrile, vinylidene chloride, vinylidene fluoride, vinylidene cyanide, vinyl chloride, vinyl fluoride, tetrafluoroethylene, etc.
Materials described in U.S. Pat. No. 5,952,130 and U.S. Ser. No. 09/354,209 provide excellent protection to imaging materials compared to those described in U.S. Pat. Nos. 5,376,434 and 5,447,832 in terms of fingerprint-resistance, However, they have a problem with respect to image discolorization due to the de-hydrochlorination of the vinylidene chloride comonomer. This phenomenum is especially noticeable in the low image density area of the image recording element. Therefore, there is a need to identify a new class of materials that does not discolorize while maintaining equivalent fingerprint resistance.
U.S. Pat. No. 6,087,051 discloses the use of polyurethane resins containing a polycarbonate expressed by the general formula of --(O--R--O--CO)--, where R is a divalent group, for forming a protective coating layer for information recording materials. The protective coating layer was described as glossy and having excellent water and fingerprint resistance. However, the materials exemplified in the examples of this patent employ a system of coating involving relatively high drying temperatures.
In view of the above, there is a need for overcoat materials that provide good film formation during coating and drying, but which provide good properties in the final coating. The overcoat material should not discolorize while maintaining equivalent water, scratch, and fingerprint resistance. It would be desirable for such a coating to be applied to an image recording element under advantageous process conditions, such that minimal or no changes to existing photofinishing systems are necessary.